Spectral dichroic dyes

Polarization films are fabricated by embedding dichroic dyes in plastic. A special process is employed to obtain uniform alignment of the dye molecules in the substrate material. The film then only absorbs light with a defined direction of oscillation. If the dye is black the polarizer is neutral , with any other dye the polarizer is selective . If for instance a red dye is embedded in the film, the filter is transparent to red light of both directions of oscillation the complementary spectral range is polarized. 

To make a black mixture, many dichroic dyes covering the whole visible spectrum are required [55-58]. Besides the desired Amax and color, the three important criteria for choosing the dyes are their stability, solubility, and order parameter. Broad spectral band dyes are preferred over narrow band dyes. 

Both spectral and polarization discrimination of the background subtracted signals are trivial to implement (see Figure 3.16). Standard dichroic filters are available for all common dye pairs and inexpensive polarization beam splitters are commonly available from optical component manufacturers. 

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